The present invention relates to total air temperature (TAT) probes or sensors. More particularly, the present invention relates to improving anti-icing performance and reducing deicing heater error (DHE) in TAT probes.
Modern jet powered aircraft require very accurate measurement of outside air temperature (OAT) for inputs to the air data computer and other airborne systems. For these aircraft types, their associated flight conditions, and the use of total air temperature probes in general, air temperature is better defined by the following four temperatures: (1) Static air temperature (SAT) or (Ts), (2) Total air temperature (TAT) or (Tt), (3) recovery temperature (Tr), and (4) measured temperature (Tm). (1) Static air temperature (SAT) or (Ts) is the temperature of the undisturbed air through which the aircraft is about to fly. (2) Total air temperature (TAT) or (Tt) is the maximum air temperature that can be attained by 100% conversion of the kinetic energy of the flight. The measurement of TAT is derived from (3) the recovery temperature (Tr), which is the adiabatic value of local air temperature on each portion of the aircraft surface due to incomplete recovery of the kinetic energy. Temperature (Tr) is in turn obtained from (4) measured temperature (Tm), which is the actual temperature as measured, and which differs from recovery temperature because of heat transfer effects due to imposed environments. For measuring the total air temperature, TAT probes are well known in the art.
Conventional TAT probes, although often remarkably efficient as a TAT sensor, sometimes face the difficulty of working in icing conditions. During flight in icing conditions, water droplets, and/or ice crystals, are ingested into the TAT probe where, under moderate to severe conditions, they can accrete around the opening of the internal sensing element. An ice ridge can grow and eventually break freexe2x80x94clogging the sensor temporarily and causing an error in the TAT reading. To address this problem, conventional TAT probes have incorporated an elbow, or bend, to inertially separate these particles from the airflow before they reach the sensing element. These conventional TAT probe designs can be very effective at extracting particles having diameters of 5 microns or greater. However, the process of particle extraction becomes increasing less efficient in many conventional TAT probe designs when removing particles below this size.
Another phenomena which presents difficulties to some conventional TAT probe designs has to do with the problem of boundary layer separation, or xe2x80x9cspillagexe2x80x9d, at low mass flows. Flow separation creates two problems for the accurate measurement of total air temperature. The first has to do with turbulence and the creation of irrecoverable losses that reduce the measured value of total air temperature. The second is tied to the necessity of having to heat the probe in order to prevent ice formation during icing conditions. Deicing and Anti-icing performance are facilitated by heater elements embedded in the housing walls. Unfortunately, external heating also heats the internal boundary layers of air which, if not properly controlled, provide an extraneous heat source in the measurement of total air temperature. This type of error, commonly referred to as DHE (Deicing Heater Error), is difficult to correct for. In conventional TAT probes, the inertial flow separation bend described above has vent, or bleed, holes distributed along its inner surface. The holes are vented to a pressure equal to roughly that of the static atmospheric pressure outside of the TAT probe. In this manner, a favorable pressure difference is created which removes a portion of the boundary layer through the bleed holes, and pins the remaining boundary layer against the elbow""s inner wall.
In certain situations, the differential pressure across the bleed holes can drop to zero due to the higher flow velocity along the elbow""s inner radius. This stagnation of flow through the bleed holes creates a loss in boundary layer control. The resulting perturbation, if large enough, can cause the boundary layer to separate from the inner surface and make contact with the sensing element. Because the housing walls are heated, so is the boundary layer. Hence, any contamination of the main airflow by the heated boundary layer will result in a corresponding error in the total air temperature measurement. In general, it is difficult to prevent the stagnation of some of the bleed holes. Thus, DHE is difficult to prevent or reduce.
A total air temperature probe positionable on a surface of an aircraft for measuring total air temperature includes an inlet scoop which receives airflow from free stream airflow moving toward the inlet scoop from a first direction. A first portion of the airflow entering the inlet scoop exits the probe through a main exit channel. A second portion of the airflow enters a TAT sensor flow passage, which extends longitudinally along an axis. This axis is oriented to form an angle of less than 90 degrees with the first direction from which the free stream airflow moves toward the inlet scoop. A sensor assembly extends longitudinally in the sensor flow passage and measures a total air temperature of airflow through the sensor flow passage. By increasing the angle through which the internal air turns, better inertial extraction of ice and water particles is realized. As a result, sensor clogging from accreted ice is significantly reduced.
A second improvement is achieved by repositioning the sensor element to be more in-line with the internal air flow direction. This helps lower DHE by minimizing heated boundary layer spillage onto the sensing element.